Examples of compounds having a diaminopyrimidine skeleton (hereinafter, referred to as “diaminopyrimidine derivatives”) include minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide), and compounds having a diaminopyrimidine skeleton are known as compounds having therapeutic and prophylactic activity on male hormonal alopecia. In Japanese Patent Application Laid-Open (JP-A) No. 2004-155689 and JP-A No. 2004-155690, there is a disclosure that diaminopyrimidine derivatives are dissolved in water solvents using 50% or more of ethanol because diaminopyrimidine derivatives have low solubility in water.
However, with respect to compositions containing ethanol at 50% or more, there is a concern about, for example, irritation to the scalp by ethanol, as is disclosed in Contact Dermatitis 1994, 30, 1-6 and The Journal of Investigative Dermatology 1977, 69, 219-222. In view of this, liquid compositions in which a specific solubilizer is contained for the purpose of solubilizing minoxidil and in which the content of ethanol is set to be less than 50% are known. As such a solubilizer, for example, bile acid or a salt thereof is proposed in JP-A No. 2010-180132, and a multimeric protein or the like is proposed in JP-A No. 2010-222283.
Moreover, various pharmaceutical preparations which exert an excellent hair-restoring or hair-growing effect through external application of minoxidil have been proposed.
For example, in JP-A No. 2011-051980, an emulsion or micelle preparation that contains a higher fatty acid, polyoxyethylene hydrogenated castor oil, a lower alcohol, and a polyhydric alcohol is proposed in order to enhance the delivery of minoxidil to the trichocyst. Here, with respect to the pharmaceutical preparation described in JP-A No. 2011-051980, it is described that lower alcohols are required for inclusion of minoxidil in a high concentration.
In JP-A No. 2005-519887, a preparation containing a lipophilic component and a hydrophilic component is proposed in which a higher fatty acid and a component such as triethanolamine, monopropylene glycol, glycerol, or polyvinylpyrrolidone are contained in order to enhance the penetration of minoxidil. In addition, in JP-A No. 2005-519887, a preparation in which propylene glycol is contained substantially in an amount of as much as 28% by weight is disclosed. Propylene glycol is known to cause strong skin irritation, as disclosed in Contact Dermatitis 2005, 53, 247-259.